1. Field of the Invention
The invention relates to a novel method, system, sensor and sensor/guide wire assembly for measuring physiological variables inside the body of a human or an animal. More particularly, it relates to a wireless sensor having a diaphragm comprising elements responsive to resonance.
2. Description of Related Art
Intravascular measurements of pressure, flow, and temperature via sensors mounted to guidewires, as disclosed in U.S. Pat. No. 5,226,423, Tenerz, reissued as Re 35,648, are of particular interest to the health care industry, and serve as tangible examples of embodiments of system solutions, device designs, components and materials to be used in the realization of the invention.
Implantable devices for physiological measurements and therapy have been used for decades, for example, in combination with cardiac pacemakers. These device carry their own power supply, are relatively bulky, and require a surgical procedure to be implanted into the patient's body.
For short term diagnostic procedures, such as intravascular measurements of pressure, flow, and temperature, the current state of the art utilizes micromechanical sensor elements, mostly silicon devices based on well known piezoresistive, capacitive, or fiber optic principles. The devices communicate information and power along a guidewire via thin optical fibers or insulated electrical leads. The guidewire is extremely small, preferably with an overall diameter no larger than 0.4 mm. Housing multiple wires in a guide wire with such an extremely small area is very difficult from a technological standpoint and is also very costly. Other difficulties arise, including, maintaining the structural integrity and the mechanical properties of the guidewire while encompassing within the guide wire the power and information transmission media. Moreover, weak points, in terms of sensor performance (e.g. zero point stability) and mechanical reliability, exist at the connecting points between the sensor and the information and power transmission wires.
A wirless system for recording pressure in the eye is disclosed in the following articles: "Passive Silicon Transensor Intended for Biomedical, Remote pressure Monitoring" by Backlund, Y. et al in Sensors and Actuators, A21-A23(1990) 58-61, Elsevier Sequoia; "A System for Wireless Intra-ocular Pressure Measurements Using Silicon Micromachined sensor" by Rosengren, L. et al in J. Of Micromech. Microeng. 2(1992) 202-204, IOP Publishing Ltd; and "A system for Passive Pressure Implantable Pressure Sensors" by Rosengren, L. et al in Sensors and Actuators A, 1994, Elsevier Sequoia. The disclosures of these publications are hereby incorporated in this specification by reference.
The wireless system disclosed in these articles comprises an electromagnetic sensor device of an L-C circuit type, having the capability of resonating in response to an excitation by an externally applied electromagnetic field, and to change its resonance frequency in reponse to a pressure change. The sensor is implanted/inserted into the eye, and excitation energy is applied. The shift in resonance frequency due to change in pressure exerted on the device is registered.
The difficulties suggested in the preceding are not intended to be exhaustive but rather are among many which tend to reduce the desirability of the known devices. Other noteworthy problems may exist; however, those presented above should be sufficient to demonstrate that such methods and apparatuses appearing in the past will admit to worthwile improvement.